Pot heat exchanger

ABSTRACT

A raw gas collection system for collecting raw gas from a plurality of aluminium smelting pots is equipped with a plurality of branch ducts, each of which is arranged to channel a respective branch flow of raw gas from an aluminium smelting pot to a collection duct, which is common to and shared by the branch ducts. Each of said branch ducts is, near an outlet thereof, equipped with a curved section for aligning the branch flow with a flow direction of raw gas already present in the common collection duct, and a constriction for accelerating the branch flow through the branch duct outlet into the common collection duct. Furthermore, each of said branch ducts is equipped with a heat exchanger for removing heat from the respective branch flow of raw gas. The combined flow resistance of the constriction and the heat exchanger reduces the need for adjusting the respective branch flows using dampers, thereby reducing the power required to transport the raw gas.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional application claiming priority to U.S. applicationSer. No. 13/824,950 having a Filing Date of Apr. 16, 2013, now U.S. Pat.No. 9,360,145 issued on Jun. 7, 2016, which is a 371 of InternationalApplication No. PCT/IB2011/002033 having an International Filing Date ofSep. 1, 2011, claiming priority to EP Application No. 10177366.1 havinga Filing Date of Sep. 17, 2010, each incorporated herein in its entiretyby reference.

FIELD OF THE INVENTION

The present invention relates to a raw gas collection system forcollecting raw gas from a plurality of aluminium smelting pots. Theinvention also relates to a method of moving a branch flow of raw gasfrom an aluminium smelting pot to a common collection duct.

BACKGROUND

Aluminium is often produced by means of an electrolysis process usingone or more aluminium production electrolytic cells. Such electrolyticcells typically comprise a bath for containing bath contents comprisingfluoride containing minerals on top of molten aluminium. The bathcontents are in contact with cathode electrode blocks and anodeelectrode blocks. Aluminium oxide is supplied on regular intervals tothe bath via openings at several positions along the center of the celland between rows of anodes.

The electrolytic reaction taking place in the electrolytic cellsgenerates a hot effluent gas that comprises gaseous components that areundesirable in the atmosphere, including hydrogen fluoride, sulphurdioxide, and the like. The process also generates fine dust. Theundesirable gaseous components and dust must be disposed of in anenvironmentally conscientious manner; hence, the raw gas is collected atthe electrolytic cells and transported to a gas cleaning unit, where anyundesirable components are preferably removed as efficiently aspossible. Dust and gaseous components such as hydrogen fluoride may bereturned to the aluminium production cells, where they may be of benefitto the production process.

A typical gas cleaning unit may comprise a dry scrubber and a dustfilter, e.g. a fabric filter that may be of the bag filter type. A rawgas collection system is typically arranged for collecting raw gas froma plurality of electrolytic cells, and transporting the raw gas to thecleaning unit. A consideration of such systems is that energy-consumingfans are often necessary in flue gas treatment systems to actively drawthe raw gas from the electrolytic cells and through the gas cleaningunit. Such is the case since the raw gas collection ducts and the gascleaning unit may introduce flow resistance in the flue gas collectionand cleaning systems.

WO 03/001106 discloses a duct system for transporting raw process gasfrom a plurality of individually located aluminium smelting pots to acentral gas filtering plant. WO 03/001106 addresses the energyconsumption of gas transport by providing each branch duct, where itopens into the common collection duct, with a narrowing outlet sectionthat is parallel to the common collection duct gas flow, so as todischarge the branch gas flow into the common collection duct at a speedthat is higher than the speed of a gas flow in the common collectionduct. Thereby, less energy is required for transporting the gas. Thereis however a need for even further reducing the amount of energyrequired for transporting raw gas.

SUMMARY

According to aspects illustrated herein, the above drawbacks anddeficiencies of the prior art are at least partially overcome oralleviated by the subject raw gas collection system for collecting rawgas from a plurality of aluminium smelting pots. The subject raw gascollection system comprises a common collection duct for passage of acommon collection duct flow of raw gas in a common collection duct flowdirection; and a plurality of branch ducts, each of said branch ductshaving an inlet fluidly connected to a smelting pot for drawing arespective branch flow of raw gas therefrom, and a discharge end fluidlyconnected to the common collection duct. The discharge ends of thebranch ducts are each equipped with an alignment section for directingthe direction of the branch flow with that of said common collectionduct flow, a discharge aperture, and a constriction for accelerating thebranch flow through the discharge aperture into the common collectionduct. Each of at least two branch ducts of said plurality of branchducts is also equipped with a heat exchanger. Each of said heatexchangers is equipped with a heat transfer element located in the flowpath of the respective branch flow, for transferring heat from therespective branch flow of raw gas to a heat transfer medium, and forgenerating a flow resistance in the respective branch duct. Such flowresistance serves to balance the velocity of the branch flows of the atleast two branch ducts. Each heat exchanger is associated with a flowresistance, and hence induces a pressure drop thereacross. The sameapplies to the constrictions in each of the discharge ends. The pressuredrop across each of the heat exchangers operates, in combination withthe pressure drop across each of the constrictions, so as to moreuniformly balance the branch flow rates/velocities between the at leasttwo branch ducts. Thereby, more predictable process conditions may beobtained in the respective smelting pots, since the supply of alumina tothe smelting pots, as well as the aluminium production process as such,depend on e.g. the composition and flow of gas inside the smelting pots,and on the temperature and pressure of said gas. Furthermore, aconstriction and a heat transfer element in each branch duct increasesthe branch duct flow resistance, thus reducing the need for e.g. branchduct regulation dampers. Duct regulation dampers are often used toregulate pressure along the common collection duct so as to achieve amore consistent/uniform pressure therein. Reducing the need for ductregulation dampers reduces the total energy consumption of the raw gascollection system.

According to an embodiment, the heat exchanger and constriction withineach of said at least two branch ducts are configured to, when in use,together generate at least 50% of a total raw gas pressure drop from therespective branch duct inlet to the respective discharge aperture. Suchresults in an even more uniform balancing of branch flows between thebranch ducts. Furthermore, the greater the pressure drop generated bycomponents having an additional function, such as exchanging heat oraccelerating gas, the less energy is required to move the gas throughthe system. By creating pressure drops using such “double function”components in the at least two branch ducts, i.e. between the inlets andthe discharge apertures of the respective branch ducts, “sole function”pressure drop generating components, such as dampers, may be eliminatedfrom the system.

According to an embodiment, the constriction in each of said at leasttwo branch ducts is provided with an adjustable flap for controlling theacceleration of the respective branch flow into the common collectionduct. Such increases the effectiveness of the acceleration, whilereducing the need for an additional regulation damper in the branch ductfor fine-tuning flow resistance within the branch duct.

According to an embodiment, the heat transfer element of each of said atleast two branch ducts is located at the discharge aperture. Thereby,the constriction for accelerating the raw gas is formed by the heattransfer element itself. The kinetic energy of the gas in the heatexchanger will thereby be better preserved on entry into the commoncollection duct.

According to an embodiment, at least one of said at least two branchducts is provided with a regulation damper for fine-tuning flowresistance within the respective branch duct. The flow rates of therespective branch flows can thereby be adjusted with a greater accuracy.

According to an embodiment, said at least two branch ducts comprise atleast 30% of all branch ducts fluidly connected to said commoncollection duct. By providing a significant number of branch ducts withheat exchangers, more uniform raw gas flow rates from each of the branchducts associated with the common collection duct may be obtained.Furthermore, if a significant number of branch ducts are equipped withheat exchangers, a balancing effect on the raw gas flow rates in thedifferent branch ducts results to somewhat equalize the raw gas flowrates through any branch ducts without heat exchangers.

According to an embodiment, said at least two branch ducts with heatexchangers are fluidly connected to the common collection ductdownstream, with regard to the flow of gas, of a plurality of branchducts not equipped with heat exchangers. This is the most effectivelocation of branch ducts with heat exchangers/increased flow resistance,since the raw gas flowing from the branch ducts not equipped with heatexchangers, i.e., without the associated added flow resistance,naturally undergoes a pressure drop due to the relatively longerdistance required to flow through the common collection duct.

According to an embodiment, heat exchangers in said at least two branchducts comprise respective raw gas inlet chambers for receiving branchflows of raw gas, and respective pluralities of parallel, spaced apartraw gas cooling tubes. Such heat exchangers are commercially desirabledue to low levels of internal scaling and low overall system energyloss, even with the flow resistance caused thereby. Accordingly, one mayachieve low overall system energy loss, while still maintaining asufficient pressure drop to balance respective branch flow rates of rawgas. According to an embodiment, each raw gas cooling tube has a coolingtube inlet funnel for accelerating the velocity of raw gas flowing intothe cooling tube. The inlet funnels may reduce scaling and may be betteradapted for the “double purpose” of raw gas flow resistance.

According to other aspects disclosed herein, the above-noted drawbacksand deficiencies of the prior art are substantially overcome oralleviated by a method of moving a branch flow of raw gas from analuminium smelting pot to a common collection duct. The subject methodcomprises cooling said branch flow of raw gas in a heat exchanger, so asto obtain cooled raw gas. The velocity of the branch flow of raw gas isaccelerated as it enters the common collection duct in a direction offlow the same as or aligned with the direction of flow of raw gasalready flowing through the common collection duct.

According to an embodiment, said heat exchanger accelerates the velocityof raw gas branch flow as it enters into the common collection duct.

According to an embodiment, said method is performed at each of aplurality of smelting pots along the common collection duct.

According to yet other aspects disclosesed herein, the above-noteddrawbacks and deficiencies of the prior art are substantially overcomeor alleviated by the use of a heat exchanger for accelerating thevelocity of a branch flow of raw gas from an aluminium smelting pot intoa common collection duct. Thereby, less energy is consumed/required tocollect raw gas.

According to still further aspects disclosed herein, the above-noteddrawbacks and deficiencies of the prior art are substantially overcomeor alleviated by the use of a plurality of flow resistance generatingheat exchangers for balancing a plurality of branch flows in arespective plurality of branch ducts. Each of said branch ducts isarranged between a respective aluminium smelting pot and a commoncollection duct. Thereby, the individual branch flows of said pluralityof branch flows may have raw gas velocities that are more uniform orbalanced, and/or less energy may be consumed for raw gas collection.

BRIEF DESCRIPTION OF THE DRAWINGS

The above, as well as additional objects, features and advantages, willbe better understood through the following illustrative and non-limitingdetailed description of exemplary embodiments, with reference to theappended drawings in which like elements are numbered alike, wherein:

FIG. 1 is a diagrammatic top plan view of an aluminium production plant;

FIG. 2 is a schematic cross-section side view, taken along line II-II,of the aluminium production plant of FIG. 1;

FIG. 3 is a schematic cross-section top view, of the discharge end ofthe branch duct illustrated in FIG. 2;

FIG. 4 is a schematic perspective view of a heat exchanger;

FIG. 5 is a schematic cross-section top view, of an alternativeembodiment of a discharge end of a branch duct such as that illustratedin FIG. 2; and

FIG. 6 is a schematic cross-section top view, of yet another alternativeembodiment of a discharge end of a branch duct such as that illustratedin FIG. 2.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

FIG. 1 is a schematic representation of an aluminium production plant 10as seen from above. The aluminium production plant 10 comprises aplurality of electrolytic cell rooms, or pot rooms 12AB, 12CD, eachcomprising a number of aluminium production smelting pots, orelectrolytic cells, 14. The electrolytic cells 14 are arranged inpotlines in the manner well known to those skilled in the art. A potlinecomprises a plurality of electrolytic cells, connected in series in adirect current (DC) loop. FIG. 1 illustrates a first and a secondelectrolytic cell room 12AB, 12CD, with each room housing a respectivepotline 16AB, 16CD. Even though a single potline 16AB, 16CD in FIG. 1 isillustrated as housed in a single electrolytic cell room 12AB, 12CD, asingle potline 16, defined as a plurality of smelting pots that areelectrically connected in series, may just as well stretch acrossseveral electrolytic cell rooms 12. By way of example, as an alternativeto the configuration described above, electrolytic cells 14 of a potline16AB could be electrically connected in series with the electrolyticcells 14 of a second potline 16CD, so as to form a single, twice as longpotline including both 16AB and 16CD, located in two separate pot rooms12AB and 12CD.

Even though the aluminium production plant 10 of FIG. 1 is provided withtwo potlines 16AB, 16CD, an aluminium production plant 10 may typicallycomprise from 1 to 20 potlines 16 located in typically from 1 to 20 potrooms 12. And even though only a few electrolytic cells 14 areillustrated in each potline 16AB, 16CD of FIG. 1, a single potline 16may typically comprise from 50 to 200 electrolytic cells 14. The dashedlines of FIG. 1 indicate that each of the electrolytic cell rooms 12AB,12CD may comprise a number of additional electrolytic cells 14, and thatthe plant 10 may comprise additional potlines and/or electrolytic cellrooms.

The process occurring in the electrolytic cells 14 may be the well-knownHall-Héroult process, in which aluminium oxide dissolved in a melt offluorine containing minerals, is electrolysed to form aluminium. Hence,the electrolytic cells 14 function as electrolysis cells. Powderedaluminium oxide is fed to the electrolytic cells 14 via an aluminadistribution system (not shown) in a manner well known to those skilledin the art.

The electrolysis process occurring in each electrolytic cell 14generates large amounts of heat, dust particles and effluent gases,including but not limited to hydrogen fluoride, sulphur dioxide andcarbon dioxide. In this disclosure, the term raw gas denotes uncleanedgas from an industrial process, such as the hot flue gas from anelectrolytic smelting pot 14. A raw gas collection system 20 isconfigured to collect and transport the raw gas from a plurality ofelectrolytic cells 14 to a gas cleaning unit 22, which cleans the rawgas such that it can safely be released to the atmosphere via asmokestack 24. Often, fresh alumina is used in the gas cleaning unit 22for dry scrubbing of the raw gas.

Typically, a raw gas collection system 20 is configured to collect rawgas from one or two pot rooms 12AB, 12CD, and a gas cleaning unit 22 isoften connected between a mirrored pair of pot rooms 12AB, 12CDaccording to the well-known “H” configuration illustrated in FIG. 1.However, even though pot rooms 12AB, 12CD are illustrated in FIG. 1 asbeing connected to a single gas cleaning unit 22, each single pot room12AB, 12CD may be connected to multiple gas cleaning units 22.

For each pot room 12AB, 12CD served by a raw gas collection system 20,the raw gas collection system 20 comprises at least one commoncollection duct, which acts as a manifold for collecting the raw gasgenerated by a plurality of smelting pots 14. In the example illustratedin FIG. 1, each of the pot rooms 12AB, 12CD is provided with arespective first common collection duct 26A, 26D, and a respectivesecond common collection duct 26B, 26C. Each set of a first and a secondcommon collection ducts thereby form a pair of common collection ducts.By way of example, a first pair of common collection ducts 26A, 26B isprovided at the first pot room 12AB; and a second pair of commoncollection ducts 26C, 26D is provided at the second pot room 12CD. Eachof the common collection ducts 26A-D may be located inside and/oroutside the respective pot rooms 12AB, 12CD. Each common collection duct26A-D extends along a portion of its respective pot room 12AB, 12CD, andis fluidly connected to a plurality of electrolytic cells 14 via arespective plurality of branch ducts 28. A respective common collectionduct flow 27A-D of raw gas flows in each respective common collectionduct 26A-D, from the respective plurality of smelting pots 14 toward gascleaning unit 22. By way of example, a first common collection duct 26Ais fluidly connected to the interior of each of a first plurality ofelectrolytic cells 14 a-f of the pot room 12AB via a number ofrespective branch ducts 28 a-f, and draws a first common collection ductflow 27A of raw gas. The first plurality of electrolytic cells 14 a-fare positioned in parallel and fluidly connect to first commoncollection duct 26A in parallel. Each branch duct 28 a-f has a dischargeend, which will be described in greater detail with reference to FIG. 3,through which branch flows 38 a-f flow from the respective branch ducts28 a-f into the common collection duct 26A.

The second collection duct 26B of the first pair of common collectionducts 26A-B is connected to a second plurality of electrolytic cells 14in first pot room 12AB in a manner similar to that of first collectionduct 26A, i.e., via branch ducts 28. The second pair of commoncollection ducts 26C-D of second pot room 12CD, is arranged in a similarmanner to that of common collection ducts 26A-B of first pot room 12AB,mutatis mutandis.

Each pair of common collection ducts 26A-B, 26C-D, are joined andfluidly connect to a respective common collection duct outlet 30AB,30CD. Common collection duct flows 27A-B, 27C-D of raw gas flow throughrespective common collection duct outlets 30AB and 30CD toward gascleaning unit 22. By way of example, the two common collection ducts26A, 26B associated with the first pot room 12AB, join and fluidlyconnect at a first common collection duct outlet 30AB, through which afirst and a second common collection duct flow 27A-B of raw gas flow.Similarly, the first and second collection ducts 26C-D join and fluidlyconnect at a second common collection duct outlet 30CD.

Each common collection duct 26A-D channels its respective commoncollection duct flow 27A-D of raw gas in a common collection duct flowdirection toward gas cleaning unit 22. Such direction of raw gas flow isillustrated in FIG. 1 by arrows within each of the common collectionducts 26A-D. The four common collection duct flows 27A-D converge in aT-shaped header duct 32, through which the raw gas enters gas cleaningunit 22.

The raw gas collection system 20 operates by under-pressure, created bya fan 34 located downstream, with regard to gas flow, of gas cleaningunit 22. Hence, the fan downstream with regard to gas flow from raw gascollection system 20 actively draws raw gas from electrolytic cells 14,via branch ducts 28, common collection ducts 26A-D, and T-shaped headerduct 32, into gas cleaning unit 22. All smelting pots 14, the entire rawgas collection system 20, and the gas cleaning unit 22, are upstream,with regard to gas flow, of the fan 34, and are maintained at anunderpressure, as compared to atmospheric pressure, when plant 10 is inuse. Such underpressure serves to keep raw gas from leaking from thesmelting pots 14 into pot rooms 12AB, 12CD.

In order to improve the cleaning efficiency of gas cleaning unit 22, itis known in the art to provide a header duct 32 with a heat exchanger 36immediately upstream of gas cleaning unit 22. Such a prior art heatexchanger 36 placement is illustrated by dashed lines in FIG. 1. In sucha case, raw gas is cooled in heat exchanger 36 prior to it entering gascleaning unit 22. An example of a heat exchanger 36 particularlyresistant to scaling is disclosed in WO 2008/113496. The heat exchanger36 may also be provided with input and output dampers, so as to make itpossible to isolate heat exchanger 36 for service and maintenance, or toswitch over to a back-up heat exchanger as the case may be.

Each of the ducts and components contacting the raw gas as it flows fromelectrolytic cells 14 to smokestack 24 imparts a gas flow resistance,which may also be represented by a pressure drop. A pressure dropcorresponds to an energy loss, which has to be accommodated for byproviding a sufficient draw from fan 34. The pressure is the lowest justupstream of the fan 34, and increases along the gas flow path in adirection opposite to that of gas flow. The highest pressure, leastamount of draw from the fan, is in the most remote electrolytic cell 14f as illustrated in FIG. 1. Hence, the pressure varies throughout thesystem including along common collection ducts 26A-D. By way of example,the pressure in the first common collection duct 26A is the lowest atthe first common collection duct outlet 30AB and the highest at the mostremote end of the common collection duct 26A, at branch duct 28 f.

The four common collection ducts 26A-D may be of similar design andfunction. Hence, for reasons of clarity, only the first commoncollection duct 26A, and the first plurality of smelting pots 14 a-fconnected thereto, will be described in the following. It will beappreciated that the other three common collecting ducts 26B-D areequipped and function like common collection duct 26A.

Referring now to first common collecting duct 26A, and the firstplurality of smelting pots 14 a-f fluidly connected thereto via branchducts 28 a-f, each of the smelting pots 14 a-f generates raw gas.Flowing raw gas illustrated as arrows to depict branch flows 38 a-f,moves through respective branch ducts 28 a-f to common collection duct26A. Each of the branch ducts 28 a-f is provided with a respective heatexchanger 40 a-f in order to cool the respective branch flows 38 a-f. Bylocating heat exchangers 40 a-f in the branch ducts 28 a-f, any heatexchanger 36, and dampers associated therewith, positioned in headerduct 32, may be eliminated. Each of the heat exchangers 40 a-fintroduces a flow resistance, and hence also a pressure drop. Thereby,heat exchangers 40 a-f located in respective branch ducts 28 a-f have anequalizing effect on the relative flow rates/velocities of theindividual branch flows 38 a-f. Such may be easier understoodconsidering that the pressure drop across an individual heat exchanger40 a-f represents a minimum total pressure drop across the entire branchflow path consisting of the heat exchangers 40 a-f plus their respectivebranch ducts 28 a-f. By way of example, as an extreme special case forillustrating the principle, the following description is provided.Should the underpressure of a single, first smelting pot 14 a becompletely lost, e.g. due to a severe gas leak in branch duct 28 aupstream, with regard to gas flow, of heat exchanger 40 a, the flowresistance of heat exchanger 40 a operates to maintain an underpressuredownstream, with regard to gas flow, of heat exchanger 40 a. Such flowresistance of heat exchanger 40 a ensures that enough raw gas will stillbe drawn from the other pots 14 b-f, even with the described severe gasleak. On the other hand, if no heat exchangers 40 a-f were present inbranch ducts 28 a-f, in the case of the described severe gas leak, fan34 would draw much more raw gas from branch duct 28 a, due to its lossof flow resistance as a result of the leak, while the flow rates throughnon-damaged branch ducts 28 b-f would decrease significantly.

In a similar manner, heat exchangers 40 a-f operate to more evenlybalance individual branch flows 38 a-f velocities of raw gas from theelectrolytic cells 14 a-f, also under normal operating conditions. Inparticular, the plurality of heat exchangers 40 a-f operate torelatively equalize pressure levels in common collection duct 26A. Suchequalization reduces the pressure drop from the discharge end of themost remote branch duct 28 f to the discharge end of the nearest branchduct 28 a, with respect to the common collection duct outlet 30AB. Theheat exchangers 40 a-f hence have an equalizing or levelling effect onthe pressure along the common collection duct 26A, and hence the flowrates of the respective branch flows 28 a-f, even if the flow resistanceis identical in each of the heat exchangers 40 a-f. More predictableprocess conditions in the smelting pots 14 a-f may thereby be obtained,which may result in more efficient aluminium production. Furthermore,the risk of leaking raw gas from the smelting pots 14 a-f to theatmosphere inside pot room 12AB is reduced, since a more stableventilation of the smelting pots 14 a-f may be obtained.

It is not necessary that the branch flows 28 a-f become exactly equaldue to the presence of heat exchangers 40 a-f. In this disclosure, theterms “balancing”, “equalizing”, “levelling” or the like means to reduceany difference between respective magnitudes, but not necessarilyeliminating all difference therebetween.

Even though each of the individual heat exchangers 40 a-f introduce apressure drop in the respective branch ducts 28 a-f, the total pressuredrop in the system may be lowered as compared to having a single heatexchanger 36 in header duct 32. The reason for this is that each branchduct of at least a first set of branch ducts 28 a-d, i.e., those branchducts closest to the common collection duct outlet 30AB, would otherwiseeach require a respective damper for levelling pressure along commoncollection duct 26A, such that the branch flows 38 a-f become relativleybalanced. Placing individual heat exchangers 40 a-f in each branch duct28 a-f renders dampers superfluous, such that they may be dispensed withor kept in an open state if already present in the system. In otherwords, by creating a pressure drop where a pressure drop is naturallyneeded using heat exchangers 40 a-f, two system requirements are metusing one system component, i.e., heat exchangers 40 a-f. Hence, heatexchangers 40 a-f fulfill two system requirements: a pressure drop andheat removal from raw gas. Using a damper to generate such a pressuredrop where the pressure drop is needed, still requires the use of a heatexchanger 36, e.g. in the header duct 32, to remove heat from raw gas.Heat exchanger 36 thus generates an additional pressure drop in headerduct 32, where a pressure drop is not needed.

Heat exchangers 40 a-f not only operate to balance individual branchflows 38 a-f flowing into common collection duct 26A, but also balancethe flow rate of common collection duct flow 27A with common collectionduct flows 27B-D in common collection ducts 26B-D, provided that branchducts 28 connected to those common collection ducts are also equippedwith heat exchangers 40.

As an alternative to providing all branch ducts 28 a-f of commoncollection duct 26A with heat exchangers 40 a-f, it would be possible toprovide only a first set of branch ducts 28 a-d with respective heatexchangers 38 a-d, and leave a second set of branch ducts 28 e-f withoutheat exchangers. In such a case, branch ducts 28 e-f of the second setcould be provided with dampers for generating a pressure drop, or theycould be free from dampers and heat exchangers so as to allow free flowtherethrough. The latter is particularly attractive in a configurationwhere the branch ducts 28 e-f of the second set are located farther awayfrom common collection duct outlet 30AB than branch ducts 28 a-d of thefirst set. Thereby, the pressure drop along common collection duct 26Afrom the branch ducts 28 e-f of the second set, will somewhat compensatefor the pressure drop of heat exchangers 40 a-d of branch ducts 28 a-dof the first set.

FIG. 2 is a schematic side view representation of the pot room 12ABtaken in cross section along line II-II of FIG. 1 viewing towardsmokestack 24. In FIG. 2, only one smelting pot 14 d is illustrated,even though each of the other smelting pots 14 a-c, 14 e-f are connectedto common collection duct 26A in a similar manner. Therefore, forreasons of simplicity and clarity, only smelting pot 14 d, itsarrangement in pot room 12AB, and its connection to common collectionduct 26A will be described in detail. It will be appreciated that thesubject description may apply to the entire first set of smelting pots14 a-c, and to the second set of smelting pots e-f, as the case may be.

A gas collecting hood 42 d is arranged over smelting pot 14 d, such thatany raw gas emission from electrolytic cell 14 d is collected so leakageinto pot room 12AB is minimized. An inlet 44 d of the branch duct 28 dis fluidly connected to gas collecting hood 42 d to draw raw gas fromsmelting pot 14 d and obtain branch flow 38 d of raw gas. A dischargeend 46 d of branch duct 28 d is arranged to discharge branch flow 38 dinto fluidly connected common collection duct 26A. Heat exchanger 40 d,arranged in branch duct 28 d, shares a support structure 48 d withcommon collection duct 26A.

A magnified top view of the area within dotted rectangle III of FIG. 2,is illustrated in greater detail in FIG. 3.

FIG. 3 illustrates the discharge of branch flow 38 d into commoncollection duct 26A. Discharge end 46 d of branch duct 28 d is providedwith an alignment section 50 d, which aligns the movement of branch flow38 d in the same direction as that of common collection duct raw gasflow 27A in common collection duct 26A. Discharge end 46 d is alsoequipped with a discharge aperture 52 d for discharging the alignedbranch flow into the interior of common collection duct 26A, and aconstriction 54 d for accelerating branch flow 38 d through dischargeaperture 52 d into common collection duct 26A. By accelerating branchflow 38 d in constriction 54 d, the increased speed of branch flow 38 dhas a positive effect increasing the speed of common collection duct rawgas flow 27A in common collection duct 26A. Thereby, energy consumptionby fan 34 (FIG. 1) may be reduced. Furthermore, the pressure dropgenerated by constriction 54 d operates to level branch flows 38 a-f(FIG. 1). Preferably, branch flow 38 d velocity is accelerated to a ratehigher than that of the common collection duct raw gas flow 27A, suchthat discharged branch flow 38 d pushes common collection duct raw gasflow 27A downstream. Even though discharge aperture 52 d is illustratedin FIG. 3 as being arranged for discharging branch flow 38 d in the samedirection as that of common collection duct raw gas flow 27A, thealignment of branch flow 38 d with common collection duct raw gas flow27A by means of alignment section 50 d need not be exact. Any change ofdirection of branch flow 38 d toward the flow direction of commoncollection duct raw gas flow 27A is, for the purpose of this disclosure,to be regarded as an alignment. According to an embodiment, dischargeend 46 d is adapted to discharge branch flow 38 d at an angle, withrespect to the direction of flow of common collection duct raw gas flow27A, of less than 45°.

Heat exchanger 40 d comprises a plurality of heat transfer elements 68d, which will be described in greater detail with reference to FIG. 4.The heat transfer elements 68 d are located in the flow path of branchflow 38 d, such that heat exchanger 40 d generates a flow resistance,and, when the plant 10 is in use, a pressure drop associated with thatflow resistance. Heat transfer elements 68 d also provide for a transferof heat from branch flow 38 d to a coolant, which may flow through theheat exchanger 40 d from a coolant inlet 72 d to a coolant outlet 74 d.

By providing each of at least two branch ducts 28 a-d with a respectiveheat exchanger 40 a-d and a respective discharge end 46 a-d of thistype, the combined pressure drop across the respective heat exchangers40 a-d and the respective constrictions 54 a-d results in a lowerpressure drop along the common collection duct 26A, from the most remotesmelting pot 14 f to the first common collection duct outlet 30AB.Likewise, such achieves a more well-balanced distribution of the flow ofraw gas between all individual branch flows 38 a-f. Furthermore, eachpressure drop generated by either heat exchanger 40 d or constriction 54d results in an additional benefit, apart from generating a pressuredrop, such as the exchange of heat with a coolant, or increasing thespeed of the common collection duct flow of raw gas in the commoncollection duct 26A. Thereby, the number of flow resistance generatingcomponents, such as dampers, that have no function other than generatinga pressure drop, may be reduced. This may result in a reduction of totalenergy consumption by fan 34, as described above.

Branch duct 28 d may also be provided with a flow regulation damper 56 dlocated upstream, with regard to the flow of gas, of heat exchanger 40d, for fine-tuning the flow resistance in branch duct 28 d. Flowregulation damper 56 d may also be used to individually isolateelectrolytic cell 14 d, in the event electrolytic cell 14 d is in needof service. A similar damper (not shown) may also be arranged in branchduct 28 d downstream, with respect to the flow of gas, of the heatexchanger 40 d so heat exchanger 40 d may likewise be isolated forservice.

Turning now to FIG. 4, heat exchanger 40 d comprises a raw gas inletchamber 66 d for receiving branch flow 38 d from branch duct 28 d, and aplurality of mutually parallel, spaced apart, raw gas cooling tubes 68d. Cooling tubes 68 d are housed in a coolant housing 70 d. Coolanthousing 70 d forms a fluid-tight compartment around the plurality ofcooling tubes 68 d, thereby allowing a fluid coolant, such as water, tobe in direct thermal contact with the exterior surface 69 d of coolingtubes 68 d. Thereby, cooling tubes 68 d act as heat transfer elements.For purposes of clarity, heat exchanger 40 d of FIG. 4 is illustratedwith parts of coolant housing 70 d “broken away”. For the same purpose,heat exchanger 40 d is illustrated as having only about 40 cooling tubes68 d. However, according to an embodiment, each heat exchanger 40 d may,by way of example, be equipped with between 100 and 3000 cooling tubes68 d in order to generate a suitable balance between flow resistance andheat exchange efficiency. More typically, each heat exchanger 40 d maybe equipped with between 100 and 600 cooling tubes 68 d. Cooling tubes68 d may, by way of example, have a length of e.g. between about 80 and200 cm (centimetres), and a diameter of e.g. between about 12 and 55 mm(millimetres). More typically, cooling tubes 68 d may have a diameter ofbetween 20 and 30 mm. Steel is a suitable material for the tubes. Heatexchanger 40 d may, according to an embodiment, be configured togenerate a pressure drop of between 100 Pa (Pascal) and 800 Pa when inuse.

Coolant flows into heat exchanger 40 d via a coolant inlet 72 d providedin an upper wall 73 d of coolant housing 70 d, and flows from heatexchanger 40 d via a coolant outlet 74 d, provided in a lower wall 75 dof coolant housing 70 d. According to an embodiment, wall 73 d and wall75 d are opposed. Heat transferred to the coolant in heat exchanger 40 dmay be used elsewhere where heat may be needed, such as for heatingbuildings, desalinating sea water, or the like.

An inlet 76 d of each cooling tube 68 d is fixedly connected to acooling tube inlet plate 78 d, which forms part of coolant housing 70 d.An outlet 80 d of each cooling tube 68 d is fixedly connected to acooling tube outlet plate 82 d, which also forms part of coolant housing70 d. Apertures 81 d through cooling tube inlet and outlet plates 78 d,82 d correspond with and fluidly connect to inlets and outlets 76 d, 80d of cooling tubes 68 d. As such, raw gas may pass via cooling tubes 68d from raw gas inlet chamber 66 d of heat exchanger 40 d to thedischarge end 46 d (FIG. 3) of branch duct 28 d.

The multiple parallel cooling tubes 68 d in heat exchanger 40 d channeland accelerate the flow of raw gas along the length thereof, therebyobtaining a relatively well-directed, uniform flow there through. Theuniformity and speed of the raw gas flow results in a relatively lowdegree of scaling.

Each cooling tube 68 d is provided with a cooling tube inlet funnel 77d, i.e. a widened cooling tube inlet, fixedly connected to cooling tubeinlet plate 78 d at aperture edges 83 d, so as to extend into inletchamber 66 d. Inlet funnels 77 d accelerate the flow of raw gas enteringcooling tubes 68 d, thereby further reducing the risk of scaling insidetubes 68 d. Although inlet funnels 77 d illustrated in FIG. 4 areconical in shape, inlet funnels 77 d may be constructed in other shapes,such as for example, a bell-shape.

By positioning individual heat exchangers 40 a-f (FIG. 1) in branchducts 28 a-f, each heat exchanger 40 a-f may be designed for cooling arelatively small branch flow 38 d of raw gas. Accordingly, heatexchangers 40 a-f may be made relatively small in size while havingsuitable capacity to accommodate the intended reduced raw gas flowvolume. Relatively small sized heat exchangers 40 a-f may more easilyshare mounting structures, access platforms, lighting, inspection lids,etc., with other devices so positioned in connection with pot room 12AB(FIG. 1), such as common collection duct 26A and/or electrolytic cells14 a-f. Furthermore, the use of heat exchangers 40 a-f at branch ducts28 a-f reduces the need for “back-up” heat exchangers, as compared tothe alternative of a centrally located heat exchanger 36 (FIG. 1). Theconsequences for the operation of plant 10, in the event of a singlemalfunction at a branch duct heat exchanger 40 a-f, are not significantin comparison to a malfunction of a relatively large centrally locatedheat exchanger 36 (FIG. 1).

FIG. 5 illustrates an alternative embodiment of branch duct dischargeend 46 d, according to which heat exchanger 40 d is located at dischargeaperture 52 d of branch duct 28 d. Thereby, heat exchanger 40 d mayserve the triple functions of exchanging heat, generating flowresistance, and accelerating the flow of branch flow 38 d into commoncollection duct 26A. Such reduces the need for a separate constrictionat discharge aperture 52 d, resulting in yet another means by whichenergy consumption by fan 34 may be reduced.

FIG. 6 illustrates an alternative embodiment, wherein an adjustable flap60 d at discharge aperture 52 d allows for fine-tuning the accelerationof the branch flow 38 d, as well as for fine-tuning the pressure drop inbranch duct 28 d.

While the invention has been described with reference to variousexemplary embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiments disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

For example, a T-shaped header duct 32 has been described above. It willbe appreciated that a header duct may have any shape, or, as analternative to channeling the raw gas into the gas cleaning unit 22 viaa header duct, each common collection duct 26 may be connected directlyto a gas cleaning unit 22.

Furthermore, not all branch ducts 28 connected to a common collectionduct 26 need to be provided with an alignment section 50 d and/or aconstriction 54 d; providing only a plurality of the branch ducts 28with an alignment section 50 d and/or a constriction 54 d is sufficientfor obtaining a positive effect on the flow distribution and energyconsumption.

Heat exchangers 40 need not be of the stacked tube type describedherein; they may be of any type known to those skilled in the art.

It is not necessary that all branch ducts 28 near a common collectionduct outlet 30 be provided with a respective heat exchanger 40 in orderto obtain a suitable balancing of branch flows 38; as an exemplaryalternative, a few selected branch ducts 28 may be provided with heatexchangers 40, and the pressure drop across the remaining branch ducts28 may be controlled in any other manner, e.g., by means of a damper.

Branch duct heat exchangers 40 a-f may be used for levelling thepressure in a common collection duct 26A regardless of the presence ofany heat exchanger 36 in header duct 32.

The invention claimed is:
 1. A method for collecting raw gas from aplurality of aluminium smelting pots with a raw gas collection system,comprising: channeling a common collection duct flow of raw gas in acommon collection duct flow direction within a common collection ducttoward a gas cleaning unit with a suction fan arranged downstream of thegas cleaning unit; drawing respective branch flows of raw gas from aplurality of branch ducts each having an inlet connected to a respectivesmelting pot of the plurality of aluminium smelting pots, and adischarge end connected to the common collection duct, said dischargeend equipped with an alignment section for aligning the respectivebranch flows of raw gas with said common collection duct flow direction,an adjustable discharge aperture, and a constriction for acceleratingthe respective branch flows of raw gas through the discharge apertureinto the common collection duct; providing each of at least two branchducts of said plurality of branch ducts arranged closest to the commonbranch duct with a respective heat exchanger, each said respective heatexchanger comprising a respective heat transfer element located in therespective branch flows of raw gas, and at least two branch ducts ofsaid plurality of branch ducts without heat exchangers; generating aflow resistance in each of the at least two branch ducts with therespective heat transfer element; balancing with the generated flowresistance raw gas flow in each of the at least two branch ducts withthe respective heat exchanger, with said respective branch flows of rawgas in said at least two branch ducts without heat exchangers; andtransferring heat from the respective branch flows of raw gas to a heattransfer medium.
 2. The method according to claim 1, further comprising:accelerating with the constriction the respective branch flows of rawgas into the common collection duct in a direction aligned with thecommon collection duct flow direction.
 3. The method according to claim1, wherein each said heat exchanger accelerates the respective branchflows of raw gas into the common collection duct.
 4. The method of claim1, further comprising: fine-tuning the generated flow resistance using aregulation damper in at least one of the at least two branch ducts.